Long-range communication of species identity by song in the Red-winged Blackbird

Summary

Adaptations of Red-winged Blackbird (Agelaius phoeniceus) song for communication of species identity over long distances were studied in an upland pasture in Ithaca, New York. This song consists of a series of brief introductory notes followed by a longer concluding trill. The trill contains energy from 2–6 kHz. Sound transmission properties and ambient noise characteristics of the habitat, changes in song composition with propagation, and the behavioral relevance of observed changes in song structure were analyzed.

• 1. At a height of 1.5 m above the ground in the habitat studied, sound transmission is greates below about 4.5 kHz. Attenuation of sound energy increases monotonically above this frequency (Fig. 3). Peak redwing song energy occurs from about 2.5–4.0 kHz (Fig. 1).

• 2. With increasing distance from a speaker most or all of the spectral structure in the introductory song components, as well as higher frequencies in the concluding trill, is severely attenuated. Songs of different males converge with distance upon a narrow frequency band (2.5–4.0 kHz) contained within the trill (Fig. 4).

• 3. This frequency band coincides with a relatively quiet region of the acoustic environment, bordered on the low frequency side by wind disturbance and, in the later part of the breeding season, on the higher frequency end by calls of various orthopteran species (Fig. 6). There may be, then, a ‘noise window’ for optimal long-distance song transmission in this spectral region.

• 4. The 2.5–4.0 kHz section of the trill is both necessary and sufficient to evoke a strong species-specific behavioral response, and contains sufficient information to do so even after transmission over a distance of at least 100 m. Neither the introductory components by themselves nor the high frequency part of the trill (4–6 kHz) are effective in producing such a response.

It is concluded from these results that the trill portion of the redwing song encodes species identity and is adapted to communicate this information to receivers at relatively long distances. Three potential functions for such an adaptation are discussed.